Polyacetylenes are structurally simple polymers having alternating double bonds along the main chain (i.e., --(CR.dbd.CR').sub.n --). Polyacetylenes are of interest due to the fact that they often possess the following properties: i) electrical conductivity (semiconductivity), ii) paramagnetism, iii) chain stiffness, iv) geometrical isomerism, and v) color.
Shirakawa and co-workers stimulated interest in polyacetylenes in the 1970 s when they reported a 10.sup.11 -fold increase in electrical conductivity of polyacetylene film upon doping. That finding stimulated interest in conjugated polymers. Researchers sought to improve their understanding of such polymers and explored new synthetic routes.
Driven by a vision of doped polymers as versatile, abundant and lightweight "plastic metals", researchers originally focused on increased electrical conductivity. More recently, conjugated polymers have been shown to have a wider range of uses, e.g., non-linear optical waveguides, light-emitting diodes, gas separation membranes, chiral separation membranes, and cell growth media.
Polymerization of substituted acetylenes has been attempted using radical initiators (e.g. 2,2'-azobisisobutyronitrile, ["AIBN"]) and ionic initiators (e.g., n-BuLi). In most cases, however, linear oligomers with low molecular weight (i.e., M.W. of about several thousand) are produced and cyclotrimers are often formed as by-products. Thus, the selective synthesis of polymers having M.W.s higher than about ten thousand proved difficult. ##STR1##
Ziegler-Natta catalysts (typically obtained by mixing an alkyl or aryl of a metal from Group I-IV of the Periodic Table with a compound, commonly a halide, of a transition metal of Group IV-VII) have been used for the polymerization of substituted acetylenes. In fact, prim- or sec-alkylacetylenes yield high molecular weight polymers in the presence of Ziegler-Natta catalysts such as a mixture of iron trisacetylacetonate and triethylaluminum [Fe(acac).sub.3 -Et.sub.3 Al(1:3)]. However, insoluble polymers and/or oligomers are produced from aromatic or heteroatom-containing monosubstituted acetylenes; and no disubstituted acetylenes are known to polymerize with Ziegler-Natta catalysts.
TABLE 1 Polymerization of HC.ident.CR by Ziegler-Natta catalysts..sup.a HC.ident.CR R Product Aliphatic Et, n-Bu, sec-Bu Soluble high polymer Aromatic Heteroatom-containing ##STR2## insoluble polymer + oligomer .sup.a RC.ident.CR': no polymerization.
The results of the polymerization of phenylacetylene (PA) by the use of conventional radical, cationic, anionic and Ziegler-Natta initiators are distinct and are summarized below.
TABLE 2 Polymerization of HC.ident.CPh by conventional initiators Initiator Example M.sub.n Radical Heat 500-2000 Cationic AlCl.sub.3 500-1500 Anionic n-BuLi -1000 Ziegler-Natta TiCl.sub.4 -Et.sub.3 -Al 400 catalyst VO(sal).sub.2 -Et.sub.3 -Al 7500.sup.a Fe(acac).sub.3 -Et.sub.3 Al 4060.sup.a .sup.a Mostly insoluble
Recently, metathesis catalysts have been used in polymerizing alkynes. The metathesis catalysts polymerize a wider range of monomers. Masuda and co-workers reported that while Ziegler-Natta catalysts can only produce high polymers from sterically undemanding acetylene or n-alkyl terminal alkynes, no soluble high polymer would be produced from alkyl alkynes with a tertiary substituent, aryl alkynes, and disubstituted alkynes. In contrast, Group V and VI metathesis catalysts polymerize more sterically demanding alkynes, such as tert-butylacetylene.
The polymerization of substituted acetylenes initiated by group V, VI and VIII transition metal complexes has also attracted a great deal of attention. In particular, Rh(I) complexes exhibit high reactivity with alkynes and have an ability to effect stereocontrolled polymerization.
A stereoregular polymer, according to the International Union of Pure and Applied Chemistry (IUPAC) definition, is a "macromolecule that can be described in terms of only one species of stereorepeating unit, in a single sequential arrangement." A stereorepeating unit is a configurational unit having a defined configuration at all sites of isomerism in the main chain of a polymer molecule.
Some Rh(I) complexes polymerize PA and its derivatives to achieve highly stereoregular substituted polyacetylenes. These include [Rh(diene)Cl].sub.2, [Rh(diene)(N--N) ]X, Rh(cod) [C.sub.5 H.sub.4 N-2-(CH.sub.2).sub.2 P(C.sub.6 H.sub.5)(CH.sub.2).sub.3 ZR]PF.sub.6, and Rh(C.sub.6 H.sub.5)(nbd)[P(C.sub.6 H.sub.5).sub.3 ].sub.2, where the term "diene" includes 1,5-cyclooctadiene ("cod") and 2,5-norbornadiene ("nbd"); "N--N" includes nitrogen-based bidentate ligand; "X" includes PF.sub.6, ClO.sub.4, and B(C.sub.6 H.sub.5).sub.4); and "ZR" includes --OC.sub.2 H.sub.5, --OC.sub.6 H.sub.5, --NH(C.sub.6 H.sub.5), and --NH(cycloC.sub.6 H.sub.11)).
Stereoregular PPA and derivatives are considered important as model compounds of ferromagnetic polymers, non-linear optical materials, and oxygen permeable materials where the geometrical structure of the main chain has to be controlled in order to draw useful properties from them.